This invention relates to a photographic element comprising at least one light sensitive silver halide emulsion layer which has enhanced photographic sensitivity.
The use of fragmentable electron donor (FED) compounds to enhance the sensitivity of silver halide emulsions has been demonstrated to be quite effective. The FED compounds have been proven to provide a sensitizing effect alone or in combination with spectral sensitizing dyes. Fragmentable electron donating compounds are described in U.S. Pat. Nos. 5,747,235 and 5,747,236 and 5,994,051, 6,010,841, 6,054,260 and 6,153,371, the entire disclosures thereof are incorporated herein by reference. These FED compounds can donate two electrons and are fragmentable, i.e., they undergo a bond cleavage reaction.
While the use of fragmentable two-electron donors increases the speed of the emulsions disclosed in the above mentioned patents, these speed increases are sometimes accompanied by unwanted fogging of the emulsion. It is desirable to have silver halides sensitized with FEDs without an increase in fog. SUMMARY OF THE INVENTION
We have now discovered that compounds that release fragmentable electron donors can improve sensitivity of photographic emulsions. In this application we describe fragmentable two-electron donor compounds that are attached to a protective group wherein such protective group is eliminated during development to give the fragmentable electron donor. The protected FED is effectively inert during the preparation, coating, and storage but becomes activated during processing of the photographic element. It is desirable to attach such a protective group to the FED so that the beneficial sensitizing effects can be obtained with lower amounts of unwanted fog.
In accordance with this invention, a silver halide emulsion layer of a photographic element is sensitized with a compound that releases a fragmentable electron donor moiety that upon donating an electron, undergoes a bond cleavage reaction to give a reducing radical. The term xe2x80x9csensitizationxe2x80x9d is used in this patent application to mean an increase in the photographic response of the silver halide emulsion layer of a photographic element. The term xe2x80x9csensitizerxe2x80x9d is used to mean a compound that provides sensitization when present in a silver halide emulsion layer.
One aspect of this invention comprises a photographic element comprising at least one silver halide emulsion layer in which the silver halide is sensitized with a compound of the formula (a), (b) or (c):
xcex94xe2x88x92(t)mxe2x88x92XYxe2x80x2xe2x80x83xe2x80x83(a)
XYxe2x80x2xe2x88x92(t)mxe2x88x92xcex94xe2x80x83xe2x80x83(b)
xcex94xe2x88x92(t)mxe2x88x92XYxe2x80x2xe2x88x92(t)mxe2x88x92xcex94xe2x80x83xe2x80x83(c)
wherein xcex94 is protective group that is eliminated during development of the photographic element, t is a timing group, m is an integer from 0 to 3, and XYxe2x80x2 is a fragmentable electron donor moiety in which X is an electron donor group and Yxe2x80x2 is a leaving proton H or a leaving group Y, with the proviso that if Yxe2x80x2 is a proton, a base, xcex2xe2x88x92, is present in the emulsion or is covalently linked directly or indirectly to X, and wherein:
1) Xxe2x80x94Yxe2x80x2 has an oxidation potential between 0 and about 1.4 V;
2) the oxidized form of Xxe2x80x94Yxe2x80x2 undergoes a bond cleavage reaction to give the radical Xxe2x80xa2 and the leaving fragment Yxe2x80x2; and
3) the radical Xxe2x80xa2 has an oxidation potential xe2x89xa6xe2x88x920.7V (that is, equal to or more negative than about xe2x88x920.7V).
It is to be understood that if Yxe2x80x2 is H, xcex94 is bonded to X, i.e., the compound is of formula (a).
In this patent application, oxidation potentials are reported as xe2x80x9cVxe2x80x9d which represents xe2x80x9cvolts versus a saturated calomel reference electrodexe2x80x9d.
This invention provides a silver halide photographic emulsion containing protected fragmentable two-electron donor compounds that release an organic electron donor capable of enhancing both the intrinsic sensitivity and, if a dye is present, the spectral sensitivity of the silver halide emulsion. An important feature of these compounds is that they contain a protective group to be eliminated at development. The protected FED is effectively inert during the preparation, coating, and storage but becomes activated during processing of the photographic element. It is desirable to attach such a protective group to the FED so that the beneficial sensitizing effects can be obtained with lower amounts of unwanted fog